Taking account of typicality in calculation of likelihood ratios

Abstract When calculating a likelihood ratio with respect to the question of whether two items originated from the same source or from different sources, one must take account of not only the similarity between the items but also their typicality with respect to the relevant population. Using simple univariate examples, this article demonstrates that likelihood ratios calculated using specific-source and common-source methods do take account of typicality, but that likelihood ratios calculated from similarity scores do not. It also demonstrates that converting feature values to percentile-rank values before calculating similarity scores does not properly take account of typicality. The article argues that methods that do not take account of typicality should not be used, and that methods that do take account of typicality should be used instead. Since sufficient case-relevant data to train a specific-known-source model are seldom available, the article recommends that the method to use instead of the similarity-score method should usually be the common-source method.